Field of the Invention
The present invention relates to a phosphate-based inorganic binder. This type of binder, used in construction products results from an acid/base reaction between a metal oxide and a phosphoric acid salt or derivative.
Description of the related art including information disclosed under 37 CFR 1.97 and 1.98
The majority of the basic oxides which react with phosphoric acid or one of its acid derivatives form, during mixing with water, a homogeneous mass which cures with time at ambient temperature. Mention may be made, among the most well known phosphate-based cements, of magnesium phosphate cements, which have been developed in order to compete with synthetic resins since they make it possible to obtain rapid-setting concretes with virtually immediate appearance of the desired strengths. They have in particular applications in the biomedical field and in dentistry, in the rapid repair of roads, tracks or bridges, or in the encapsulation of dangerous waste. A cement of this type, known as Ceramicrete, developed in the United States is obtained by mixing magnesium oxide or magnesia with potassium monophosphate KH2PO4. In the presence of water, the magnesia reacts with the phosphate groups under acid conditions to form an insoluble phosphate phase which cures and which has the properties of a ceramic. This phase is k-struvite of formula MgKPO4.6H2O. The reaction of the formation of this phase is very rapid and requires the use of a set-retarder. Boric acid is very often used to increase the workability time of this type of cement. However, according to European regulations and in particular the REACH Directive, boric acid is classified among substances which are toxic to reproduction. Other solutions for controlling the setting time are consequently envisaged. Papers describe the possibility of extending the setting time and thus the workability by using a specific magnesium oxide, namely magnesia calcined at high temperature, and by varying the size of the particles of the basic metal oxide involved. However, this does not make it possible to achieve setting times or workability times comparable to those which are obtained for systems based on Portland cements. Another disadvantage of this type of system is related to the high exothermicity of the reaction which takes place between the magnesia and the phosphate. The boric acid used as retarder does not make it possible to control the thermodynamics of the reaction. The final user thus has to take into account this high. exothermicity, which results in significant evaporation of the water present in the system during the preparation of the construction material.
Moreover, another disadvantage related to the excessively high reactivity and setting of these systems is the appearance of bleached regions at the surface of the cured material, which reflects a rise to the surface of unreacted salts. It is a matter of an efflorescence which gives a major aesthetic appearance defect which is not acceptable for the desired applications.
It is necessary to develop systems of phosphate-based binders, usable in mortar compositions, which exhibit longer workability times compatible with the desired applications and which do not exhibit the disadvantages described above. The present invention comes within this context.